Standards set by the National Fire Protection Association (NFPA) as detailed in the National Electric Code, Article 517-104 require that each power circuit within an anesthetizing location, such as in a hospital operating room, shall be ungrounded and isolated from any distribution system supplying other, non-anethetizing locations. Additionally, other articles of the NFPA have a requirement that a monitor must also be used with an ungrounded system that could be used in a "wet location" where hospital patients might be present. The monitor must provide a continuous indication of possible leakage or fault currents from any of the isolated conductors to ground. This requirement is for the safety of the patient. The monitor and related components are mounted on an isolation panel located within the anesthetizing location or hospital operating room. The normal high impedance of the human body can be bypassed during certain medical procedures such as when electrodes or probes are used to monitor heart activity, for instance. Under these conditions, alternating current flowing through the body could produce extreme shock or even death.
The ideal isolated conductor in an ungrounded system has infinite impedance to ground and no current flow would result if a short circuit or very low impedance was placed between the conductor and ground. In the real world, however, there are no perfect insulators. All isolated conductors experience some capacitive or resistive leakage current to ground. Insulation deteriorates with age and use. Capacitive leakage are inherently present in all systems. Both types of leakages increase by the number of devices connected to the system and the length of the conductors themselves. These leakages provide a current path to ground and if a grounded low impedance is connected to the isolated conductor, as might be the case with a heart monitor, the current has a return path due to the leakages. Total hazard current is defined as the total current that would flow through a low impedance conductor connected between ground and the isolated conductor. Total hazard current is a combination of fault hazard current, that current that results from all user devices, except for a LIM, connected to the isolated system, and monitor hazard current, that current that results only from a LIM connected to the isolated system. The NFPA standards require a visual and audible warning if the total hazard current exceeds a predetermined limit and also limits the amount of current attributed to the monitor.
Several types of LIMs have been available for some time. These include static ground detector and dynamic LIMs. These types, although somewhat effective, did not offer continuous monitoring, were somewhat difficult to use, and also added a large amount of hazard current to the total hazard current. One type of dynamic LIM that overcame some of these problems is disclosed in commonly assigned U.S. Pat. No. 3,976,987. A DC reference voltage, proportional to the maximum voltage to ground from either conductor of a two wire ungrounded system is determined and used as a reference voltage for the system. A capacitive component and a resistive component are generated from the reference voltage and are applied across the system leakage impedances in parallel. Circuitry within the LIM separates the impedance voltage response from the line voltage response to produce a difference signal. This signal is used to derive a combined resistive and capacitive component signal representing the maximum hazard current of the system.
If more than one isolated power circuit is within the anesthetizing location or operating room, a separate isolation panel must be used for each circuit. Operating rooms, for instance, usually require heart monitoring equipment that operates from a 120 VAC source and X-ray equipment operating at 240 VAC. This requires two isolation transformers to provide the voltages. A more effective system would be to generate these voltages from a single transformer with multiple secondaries. Prior art LIMs however, are not capable of being coupled to the same transformer at the same time due to LIM to LIM interference. It would be desirable to provide an isolation panel having line isolation monitors that overcome these disadvantages and includes other features that accurately computes the hazard current of the dual ungrounded power system.